1 Field of the Invention
The present invention relates to a polarization wave diversity optical receiver for coherent heterodyne optical communication, and more particularly, to a polarization wave diversity optical receiver utilizing a baseband combining method.
A coherent optical communication method utilizing the amplitude, frequency and phase, etc., of a light carrier having a high coherency to provide an optimum optical communication, has become widely used. A problem arises when realizing such a coherent optical communication method from the fluctuation of the state of the polarization wave, and as a countermeasure thereto, a polarization wave diversity receiving method has been proposed.
In this polarization wave diversity receiving method, two polarized wa components combined. Accordingly, in this method of combining intermediate frequencies, a method of combining basebands is considered.
The intermediate frequency combining method is a method in which the phases of two polarization wave components are previously made to coincide and are then combined by adding. The base band combining method is a method in which the two polarization wave components are demodulated and then combined by adding the two components together. In the intermediate frequency combining method, the matching of the phases is difficult, thus the realization thereof is difficult. Whereas, in the baseband combining method, although two detection circuits are necessary, the realization thereof is easy.
2. Description of the Related Art
In a conventional coherent optical communication system, a polarization wave diversity receiving system is preferably employed to overcome the signal fading caused by the fluctuation of the state of the polarization wave. To realize the polarization wave diversity receiving system, the above-mentioned baseband combining method is conventionally and preferably employed. In the conventional baseband combined method, two polarized components of intermediate frequency signals are simply combined and the combined signal is fed back to control a local oscillating circuit.
Since the phases of the intermediate frequency signals, however, do not always match each other, the power of the combined signal may often become zero.
If the power of the combined signal becomes zero, control of the local oscillating circuit is impossible. So, in the conventional baseband combining system, the stabilization of the intermediate frequency cannot always be guaranteed.